The 45-year-old actor, who plays Dr. Ben Warren joining the efforts to rescue Stephanie from an explosion.
The firefighter spinoff now stars Jaina Lee Ortiz in the lead role.
How George's character will transition from the hospital to the firehouse is still unclear as details are being kept a secret.
E!News reports that the series will debut midseason, and focus on the professional and personal lives of a group of hero firefighters in Seattle - from the captain and down the ranks.
Well, The Hollywood Reporter shared the scoop that Jason George's character is joining the spinoff, but they added that he would stay a series regular on Season 14 until production on the spinoff began; they also said his casting did not mean he couldn't continue to "pop in and out" of "Grey's Anatomy". He was promoted to series regular in season 12.
At the time the spinoff was announced, ABC originally envisioned it with an entirely new cast - though George remained a likely candidate given Ben's actions in the season-13 finale.
The new show is the second Grey's Anatomy spinoff following Private Practice, and was given a straight-to-series order in May. That episode saw Stephanie decide she'd had enough and leave Grey Sloan, and it's possible Ben will be shown to have had a similar epiphany. Shonda Rhimes and Betsy Beers will executive produce with McKee. He's repped by APA and Morris Yorn.